Enhancing methane production from food waste fermentate using biochar: the added value of electrochemical testing in pre-selecting the most effective type of biochar

Abstract

Background: Recent studies have suggested that addition of electrically conductive biochar particles is an effective strategy to improve the methanogenic conversion of waste organic substrates, by promoting syntrophic associations between acetogenic and methanogenic organisms based on interspecies electron transfer processes. However, the underlying fundamentals of the process are still largely speculative and, therefore, a priori identification, screening, and even design of suitable biochar materials for a given biotechnological process are not yet possible.

Results: Here, three charcoal-like products (i.e., biochars) obtained from the pyrolysis of different lignocellulosic materials, (i.e., wheat bran pellets, coppiced woodlands, and orchard pruning) were tested for their capacity to enhance methane production from a food waste fermentate. In all biochar-supplemented (25 g/L) batch experiments, the complete methanogenic conversion of fermentate volatile fatty acids proceeded at a rate that was up to 5 times higher than that observed in the unamended (or sand-supplemented) controls. Fluorescent in situ hybridization analysis coupled with confocal laser scanning microscopy revealed an intimate association between archaea and bacteria around the biochar particles and provided a clear indication that biochar also shaped the composition of the microbial consortium. Based on the application of a suite of physico-chemical and electrochemical characterization techniques, we demonstrated that the positive effect of biochar is directly related to the electron-donating capacity (EDC) of the material, but is independent of its bulk electrical conductivity and specific surface area. The latter properties were all previously hypothesized to play a major role in the biochar-mediated interspecies electron transfer process in methanogenic consortia.

Conclusions: Collectively, these results of this study suggest that for biochar addition in anaerobic digester operation, the screening and identification of the most suitable biochar material should be based on EDC determination, via simple electrochemical tests.

Keywords: Anaerobic digestion; Biochar; Direct interspecies electron transfer (DIET); Electron-donating capacity (EDC); Food waste; Methane.

Fig. 1

Time course of total VFAs (a), n-butyrate (b), propionate (c), and acetate (d) concentration in bottles supplemented with conductive particles and in unamended controls, during the 1st feeding cycle. Error bars represent the standard deviation of two replicate bottles

Fig. 2

Time course of methane formation yield (%) from fermentate VFAs, in bottles supplemented with conductive particles and in unamended controls, during the 1st feeding cycle. Error bars represent the standard deviation of two replicate bottles

Fig. 3

Time course of total VFAs (a), n-butyrate and i-butyrate (b), propionate (c) and acetate (d) concentration (mgCOD/L) in bottles supplemented with conductive particles, with non-conductive sand, and in unamended controls, during the 2nd feeding cycle. Error bars represent the standard deviation of two replicate bottles

Fig. 4

Time course of methane formation yield (%) from fermentate VFAs, in microcosms supplemented with conductive particles, with non-conductive sand, and in unamended controls, during the 2nd feeding cycle. Error bars represent the standard deviation of two replicate microcosms

Fig. 5

Time course of ammonia nitrogen (mg NH₃-N/L) concentration in microcosms supplemented with conductive particles, with non-conductive sand, and in unamended controls, during the 2nd feeding cycle. Error bars represent the standard deviation of two replicate microcosms

Fig. 6

Correlation between the initial methane formation rate (mgCOD/L d) and the electron donating capacity (EDC) (meq/g) (a); the electron accepting capacity (EAC) (meq/g) (b); the electrical conductivity (S/m) (c), and the specific surface area (m²/g) (d) of the different biochars. Legend: (1) wheat bran (this study); (2) wood (this study); (3) orchard (this study); (4) pine [29]; (5–6) rice straw [62]; (7–9) rice straw [63]; (10) corn stover [64]; (11) pine [64]

Fig. 7

CLSM combined images showing the spatial distribution (X–Y and Y–Z planes) of Archaea (red) and Bacteria (green) cells identified by FISH in aggregates from the unamended control (a), silica sand-supplemented control (b), wheat bran biochar (c), wood biochar (d), and orchard biochar (e). Biochar particles, visualized by their reflection signal in the same microscopic field, appear gray. Each image is composed by 32–40 optical sections of the aggregate thickness every 0.4–0.5 μm